docs: add schema type widening advice

[JstnMcBrd]

Summary

While using Mongoose with TypeScript, @ShunnShine and I discovered the dangers of defining the schema in a separate variable when we realized all of our fields with required: false were being treated as required: true.

We eventually figured out how to fix the type definitions by using as const.

We looked at the Schemas in TypeScript documentation and saw a warning about assigning a schema to a temporary variable (fair enough), but did not see any information about using as const, even though it is used in the example code right below - without explanation.

This PR adds a little section about type widening and as const to help users understand why this was done.

[vkarpov15]

Thanks 👍